1. Field of the Invention
The present invention relates to a variable gain amplifier, particularly to a radio frequency amplifier.
2. Description of the Related Art
Amplifier circuit structures the gain of which changes in response to the value of an applied control voltage are generally known. A circuit of such type is shown in FIG. 1 where it is possible to distinguish three functional blocks: a voltage/current converter 10, a current divider 11 and a current/voltage conversion load 12. The current divider 11 is typically formed by two transistors Q10, Q20, for example bipolar transistors, in differential stage configuration, which have the emitter terminals connected with each other and coupled to the output of the block 10, the collector terminal of the transistor Q10 is connected with a supply voltage Vcc and the collector terminal of the transistor Q20 is connected with the block 12, while a control voltage Vc is applied at the base terminals of the transistors Q10 and Q20. The amplification gain variation occurs in response to the application of the voltage Vc at the differential couple Q10, Q20; in fact, because of the unbalance due to the application of the voltage Vc at the transistor couple Q10, Q20, a division of the current signal is obtained according to the rule:   iout  =      in          1      +              ⅇ                  Vc          Vt                    
wherein iout is the output current of the block 11, in is the input current of the block 11 and Vt is the thermal voltage; in such case if the current gain of the block 11 is indicated by Ai1=iout/in, it is obtained that such gain is a function of the exponential type of the control signal Vc. Indicating with:   Av1  =      20    ⁢          xe2x80x83        ⁢          log      ⁡              (                  Vout          Vin                )            
the voltage gain in decibel wherein Vout is the output voltage of the whole circuit and Vin is the input voltage and assuming for example that the block 12 is constituted only by a resistor RL and the relation between Vin and in in the block 10 is Vin=Gm*in wherein Gm is the transconductance gain of the block 10, for voltage values Vc greater than  greater than Vt it is obtained:   AV1  =            20      ⁢              xe2x80x83            ⁢              log        ⁡                  (                      2            ⁢            Gm            ⁢                          xe2x80x83                        ⁢            RL                    )                      +          k      ⁢              Vc        Vt            
where K is a constant factor. Therefore the gain Av1 in decibel becomes a linear function of the control voltage Vc.
The condition Vc greater than  greater than Vt provides operatively to maintain the differential couple Q10, Q20 in high unbalance state, by making the greatest part of the signal current to pass through the electric path connected with the power supply Vcc. This represents a limit of such circuit because the maximum gain is automatically determined when the value of the load RL has been defined and a certain accuracy of the linearity characteristic of the gain curve has been assigned. For increasing the gain in the applications wherein it is required, it is necessary to act on the load RL since the value of the transconductance Gm is fixed on the base of the distortion constraints of the signal; this causes a high reduction of the amplifier band of the amplifier and a high increasing of the thermal noise. Therefore the aforementioned circuit does not result applicable in the cases wherein high gains, high bandwidth and low noise are required.
In view of the state of the art described, it is an object of the present invention to form a variable gain amplifier that overcomes at least in part the aforementioned limitations.
According to the present invention, such object is attained by a variable gain amplifier comprising a first device to which a first control signal is applied so that the gain of an output signal of said first device with respect to a first input signal is a function of the exponential type of said first control signal, characterized by comprising a feedback network connected between an output terminal and an input terminal of said first device so as to assure that said gain in decibel of said first device is a linear function of said first control signal.
Thanks to the present invention it is possible to form a variable gain amplifier that can be utilized in application requiring contemporaneously high gain, low noise and high bandwidth.